It is frequently desirable to communicate with devices located inside the human body. For instance, it may be desirable to receive information from an implanted device such as a pacemaker or implanted cardiac defibrillator. A physician may want to know the status of the device such as battery charge or pulse delivery information. It may also be desirable to send information to the implanted device from outside the body such as to reprogram or change settings of the device. It may also be desirable to communicate with a catheter while it is located in a patient's vascular system such as during the placement of a stent by the catheter. Images or measurements may need to be supplied to the attending physician during a procedure, for instance. It may also be desirable to receive information from the tip of a needle during a biopsy or ablation procedure. The information to be communicated to the physician can include data regarding needle placement or the state of surrounding material. In an ablation procedure the receipt of temperature and tissue density information is important for determining the progress of the procedure. While this information can sometimes be communicated through wires in a catheter, biopsy needle or ablation device, the small size of a catheter, needle or ablation device often affords only limited space for communication lines, particularly where wiring is also needed for the functionality of the device itself. In such situations, wireless communication will obviate the need for communication conductors through the device. But common wireless techniques such as r.f. communication are often limited by the environment in which the communication must take place. Electrical or electromagnetic transmission can be highly attenuated by the body and can encounter interference from other medical equipment such as magnetic resonance systems and pacemakers, which create a noisy environment for through-body r.f. communication. Furthermore, r.f. and electromagnetic communication can be band-limited when operating through tissue. An implantable device using an ultrasonic transducer in kHz frequency range for wireless communication is known from WO 2008/011570. The data transfer rate of such devices is limited. Accordingly it is desirable to provide a wireless through-body communication system which can operate effectively from within the body and is not band-limited.